In operation according to various wireless networks, the radio network implements control with respect to the terminals. For example, in many cellular radio communication systems (e.g., systems of long term evolution (LTE) networks, also referred to as 3GPP LTE networks, and 5th Generation (5G) or new radio (NR) networks, also referred to as 3GPP NR networks) uplink transmission parameters such as frequency, timing, and power are regulated via downlink control signaling from the base station (e.g., enhanced node B (eNB) or next generation node B (gNB)) to the terminal (e.g., user equipment (UE)).
A random access procedure, implemented using a physical random access channel (PRACH), may be used in cellular radio communication systems such as LTE and 5G/NR networks for a UE to initiate a connection with a base station. In such a random access procedure, as shown in wireless communication network 100 illustrated in FIG. 1, a UE (e.g., UE 120) may receive downlink signals providing various communication parameters, such as may include synchronization signals (e.g., primary synchronization signal (PSS) and secondary synchronization signal (SSS)), downlink and uplink carrier configuration information (e.g., master information block (MIB)), downlink control information (DCI), and initial access parameters (e.g., system information block (SIB)) broadcast by a base station (e.g., base station 110). Thereafter, the UE may select a random access preamble (e.g., a preamble with a specific bit sequence that has good auto-correlation properties) and transmit the preamble on a PRACH (i.e., transmit a PRACH signal) for detection by the base station and subsequent assignment of communication resources to the UE, such as via the physical downlink control channel (PDCCH).
Single-carrier frequency-division multiple access (SC-FDMA) may be implemented by cellular radio communication systems such as LTE and 5G/NR networks, and thus the aforementioned PRACH signal in such implementations is mixed with data and other control signals. For example, as shown in FIG. 2, PRACH 201 may be mixed with control channels and/or data channels of various UEs operating in the wireless network in multiplexed uplink 200. It should be appreciated from the illustration of multiplexed uplink 200 in FIG. 2 that in the frequency domain the PRACH is multiplexed with other channels occupying different frequency bands, whereas in the time domain the SC-FDMA signal comprises a mixture of orthogonal frequency division multiplexing (OFDM) symbols for PRACH and other channels.
In PRACH configurations implemented by cellular radio communication system such as LTE and 5G/NR networks, different subcarrier spacing may be utilized with respect to the PRACH and other channels multiplexed in the uplink. For example, as illustrated in FIG. 2, PRACH 201 may utilize different (e.g., smaller) subcarrier spacing than other channels (e.g., data and/or control channels) of multiplexed uplink 200, whereby resource grid 211 (e.g., 864×1 frequency domain resource grid for the PRACH signal) of PRACH 201 differs from resource grid 212 (e.g., 72×12 frequency domain resource grid for the data and/or control channel signals) of the other channels.
It can be appreciated that, in the random access operation described above, the base station needs to operate to detect the PRACH signal from the UE. In particular, the PRACH signal should not only be detected by the base station with high confidence, but may be used for propagation delay estimation once detected and thus should be detected with accurate timing estimation. However, due to the different subcarrier spacing of the PRACH and other channels of the multiplexed uplink, wherein the resource grid of the PRACH differs from the resource grid of the other channels, extraction of PRACH signals from the multiplexed uplink presents challenges.
In light of differences between the PRACH and other channels of the multiplexed uplink (e.g., the aforementioned different subcarrier spacing and corresponding different resource grids), a conventional uplink receiver block may include an uplink data receiver chain for extracting the data of control channels and data channels of the multiplexed uplink and an uplink PRACH receiver chain for extracting the PRACH signal of the multiplexed uplink. For example, an uplink receiver block of a base station of cellular radio communication system such as LTE and 5G/NR networks typically includes an uplink data receiver chain and an uplink PRACH receiver chain.
In uplink receiver block 300 of FIG. 3, implementing a typical configuration of uplink receiver block, it can be seen that the PRACH and other channels of the multiplexed uplink are processed separately. Uplink data receiver chain 310 is provided for extracting data (e.g., from data and/or control channels) from the multiplexed uplink. Accordingly, uplink data receiver chain 310 of FIG. 3 includes cyclic prefix (CP) removal circuit 311 for removal of cyclic prefix symbols from the received uplink signal, fast Fourier transform (FFT) circuit 312 for transforming the time domain uplink signal (except CP) to the frequency domain having the subcarrier spacing of the data and control channels (i.e., resource grid 212), and control signal processing circuit 313 and data signal processing circuit 314 for extracting the control data and payload data, respectively, from channels of the multiplexed uplink.
Uplink PRACH receiver chain 320 of uplink receiver block 300 of FIG. 3 is provided for extracting PRACH signals from the multiplexed uplink. It should be appreciated that a PRACH filter is generally needed to facilitate extracting PRACH signals from the mixture of signals in the multiplexed uplink. A combination of low pass filter (LPF) and down sampling is traditionally used with respect to time domain signals by the base station to restore the PRACH signal. A typical implementation of a PRACH filter providing time domain filtering of the PRACH signal using a combination of LPF and down sampling is shown in uplink PRACH receiver chain 320 illustrated in FIG. 3. Uplink PRACH receiver chain 320 of FIG. 3 includes low pass filter (LPF) circuit 321 for isolating the PRACH from other channels of the multiplexed uplink, down sampling circuit 322 for reducing the sampling rate of the PRACH signal (e.g., to reduce the size of FFT subsequently utilized) to provide PRACH requisite samples for PRACH signal detection, FFT circuit 323 for transforming the PRACH requisite samples to the frequency domain having the subcarrier spacing of the PRACH (i.e., resource grid 211), and PRACH detection circuit 324 for detecting the PRACH signal in the PRACH. It should be appreciated that, although LPF circuit 321 and down sampling circuit 322 are shown separately in the illustration of FIG. 3, these circuits, or a portion thereof, may be combined.
The foregoing PRACH filter is complex and requires significant computational complexity. For example, LPF circuit 321 of uplink PRACH receiver chain 320 processes all the received multiplexed uplink input samples in the raw sampling rate. Thus, significantly more samples than those of the relatively narrow band PRACH signal (e.g., 24576 samples for format 0 at a sampling rate, fs=30.72M) are processed. Moreover, because the PRACH signal bandwidth is very narrow compared to the carrier bandwidth, the PRACH filter of uplink PRACH receiver chain 320 must be very narrow. Accordingly, LPF circuit 321 illustrated in FIG. 3 includes a number of LPF filter instances and down-samplers, wherein the bandwidth of the LPF circuit is determined by fs and the subcarrier spacing and the down-sampling rate is proportional to fs (e.g., the down-sampling rate may be as large as 96 in 5G/NR networks). Such LPFs introduce extra delay and increase latency. Further adding to the complexity, different configurations of LPF may be required for accommodating multiple frequency locations for the PRACH signals (e.g., 5G/NR networks may necessitate the use of different LPF designs to accommodate various PRACH signal formats). Thus, although a single instance of LPF circuit 321 is shown in FIG. 3, multiple such LPF circuits may be implemented with respect to any particular base station.